The present invention relates to a set of novel microwave dielectric ceramic compositions xMO-yLa2O3-zTiO2 (M=Sr, Ca; x:y:z=1:2:4, 2:2:5, 1:2:5 or 1:4:9) and devices comprising the same.
The dramatic developments in the microwave integrated circuit technology have revolutionized the field of telecommunications. Dielectric resonators (DRs) are key components in MIC technology that increasingly replace the conventional metallic cavity resonators and microstrip circuits. The size of the microwave circuit is inversely proportional to the square root of its dielectric constant. A DR should have high dielectric constant (for miniaturization), high quality factor (for frequency selectivity) and low temperature coefficient of resonant frequency (for frequency stability). These constraints usually limit the DR applicability to only those ceramics with xcex5r=20-100, Q greater than 2000 and xcfx84f less than xc2x120 ppm/xc2x0 C. Compared to the use of alumina substrates, low loss high dielectric constant materials decrease the size not only for strip line resonators and filters but also for all microwave circuits. It is also possible to use these dielectric materials in the fabrication of devices such as circulators, phase shifters etc. for impedance matching. The DRs are used for the manufacture of microwave oscillators, filters and dielectric resonator antennas for satellite and personnel communication applications. They are also used for applications such as telemetry and tracking.
Conventional microwave ceramics fall into categories. (1) Ceramics with low permittivity (20 less than xcex5r less than 40) with high quality factor (Qxf greater than 50000 GHz) such as Ba(Mg,Ta)O3, Ba(Zn,Ta)O3, Ba(Mg,Nb)O3, Ba(Zn,Nb)O3, their solid solution modifications, Ba(Mg,Sn,Ta)O3, Ba2Ti9O20 and (Zr,Sn)TiO4. (2) Ceramics with high permittivity (xcex5r greater than 75) and low Q factor (Qxf less than 10000 GHz) such as tungsten bronze type BaOxe2x80x94RE2O3xe2x80x94TiO2 (1:1:4 and 1:1:5) and Ba6xe2x88x923xLn8+2xTi18O54 where Ln is La3+, Nd3+, Sm3+ and Gd3+. The first group of ceramics are usually employed at frequencies  greater than 1.5 GHz where as the second group at frequencies  less than 2 GHz. For applications at  less than 2 GHz, though ceramics with xcex5rxe2x89xa770 can provide greater miniaturization, due to the requirements of narrow bandwidth and extremely low insertion loss ( less than 0.3 dB) ceramics with even xcex5r=38 are used by compromising size. Hence further miniaturization of devices require low xcfx84f ceramics with xcex5r greater than 45 and Qxf greater than 45000 GHz. [B. Jancar, D Suvorov, M. Valent, xe2x80x9cMicrowave dielectric properties of CaTiO3xe2x80x94NdAlO3 ceramicsxe2x80x9d, J. Mater. Sci. Letters 20 (2001), 71-72]. Two different methods are used in this regard. (1) The formation of solid solutions between high Q ceramics with xcex5r in the range 20 to 40 and having reasonably high xcfx84f with ceramics of opposite xcfx84f (usually positive), high xcex5r ( greater than 100) and low loss such as CaTiO3, TiO2, SrTiO3, BaTiO3 etc. (2) Explore the microwave dielectric properties of Ba5Nb4O15 type cation deficient hexagonal perovskites. [H. Sreemoolanadhan, M. T. Sebastian and P. Mohanan, Mater. Res. Bull, 30(6) 1995, pp 653-658, C. Veneis, P. K. Davies, T. Negas and S. Bell, Mater. Res. Bull 31(5) 1996 pp 431-437 and S. Kamba, J. Petzelt, E. Buixaderas, D. Haubrich and P. Vanek, P. Kuzel, I. N. Jawahar, M. T. Sebastian and P. Mohanan, J. Appl. Phys 89(7) 2001, pp 3900-3906]. The cation deficient provskites reported such as Ba5Nb4O15 and Ba5xe2x88x92xSrxNb4O15, are having dielectric constant between 40 and 50 and high quality factor, but their high xcfx84f make them not suitable for practical applications. The isostructural BaLa4Ti4O15 and Ba2La4Ti5O18 have high dielectric constant (43 and 46), high quality factor with low temperature coefficient of resonant frequency. [C. Veneis, P. K. Davies. T. Negas and S. Bell, Mater. Res. Bull 31(5) 1996 pp 431-437] The MOxe2x80x94La2O3xe2x80x94TiO2 (M=Sr, Ca) ceramics covering the present patent consist of cation deficient perovskites MLa4Ti4O15 (M=Sr, Ca) and Ca2La4Ti5O18 belonging to the homologous series AnBnxe2x88x921O3n (5, 6 or 8) [V. A. Saltykova, O. V. Mehikova, N. F. Fedorov, Russian J. Inorg. Chem. 34(5) 1989, p 758-759] and orthorhombic structured compounds CaLa4Ti5O17 and CaLa8Ti9O31  [see JCPDS files 27-1057, 27-1058, 27-1059]. The dielectric properties of these materials are being investigated for the first time.
The main object of the present invention is to provide a set of novel microwave dielectric ceramic compositions xMO-yLa2O3-zTiO2 (M=Sr, Ca; x:y:z=1:2:4, 2:2:5, 1:2:5 or 1:4:9) and devices comprising the same which obviates the drawbacks as detailed above.
Another object of the present invention is to provide a set of novel dielectric ceramic compositions having high dielectric constant and low xcfx84xcex5 for capacitor applications.
Yet another object of the present invention is to provide a set of novel microwave dielectric ceramics having high dielectric constant and low xcfx84f for microwave substrate applications.
Accordingly the present invention provides a novel microwave dielectric ceramic composition of the general formula xMO-yLa2O3-zTiO2 wherein M is Sr or Ca; x:y:z=1:2:4, 2:2:5, 1:2:5 or 1:4:9. ps and devices comprising the same which comprises the manufacture of the inventive perovskites in the powder form, moulding of the powder in the suitable shape, drying, sintering and the final treatment.
In an embodiment of the present invention the dielectric ceramic compositions xCaO-yLa2O3-zTiO2 (x:y:z=1:2:4 and 2:2:5) are prepared in cylindrical pellet shape through the solid state ceramic route by taking high purity CaCO3, La2O3, and TiO2 in the molar ratios 1:2:4 and 2:2:5, the pellets are polished, physical, structural and microwave dielectric properties are characterized. The initial firing of the mixed powders is carried out at sufficiently higher temperatures and for sufficient durations such that single-phase polycrystalline ceramics CaLa4Ti4O15 and Ca2La4Ti5O18 are obtained.
In another embodiment of the present invention the ceramic compositions SrOxe2x80x942La2O3xe2x80x944TiO2 are prepared in cylindrical pellet shape through the solid state ceramic route from high purity SrCO3, La2O3 and TiO2 in the molar ratio 1:2:4, the mixed powders are fired at different temperatures above 1200xc2x0 C., the pellets are polished, physical, structural and microwave dielectric properties are characterized. The firing of the mixed oxide powders is carried out at sufficiently high temperatures for sufficient duration for getting single-phase polycrystalline ceramics of SrLa4Ti4O15.
In yet another embodiment of the present invention dielectric ceramic compositions xCaO-yLa2O3-zTiO2 (x:y:z=1:2:5 and 1:4:9) are prepared in cylindrical pellet shape through the solid state ceramic route, from a mixture of high purity CaCO3, La2O3, and TiO2 taken in the molar ratios 1:2:5 and 1:4:9 and by firing at temperatures greater than 1200xc2x0 C. The pellets are polished and physical, structural and microwave dielectric properties are characterized. The firing of the mixed oxide powders is carried out at sufficiently higher temperatures for sufficient duration such that single-phase polycrystalline CaLa4Ti5O17 and CaLa8Ti9O31 ceramics are obtained.
The first two embodiments comprise materials with the cation deficient hexagonal perovskite structure whereas the third one comprises materials with orthorhombic structure. The AOxe2x80x94La2O3xe2x80x94TiO2 (A=Ca, Sr) system provides ceramic materials with high dielectric constant ( greater than 40), high quality factor ( greater than 6800 at 4-5 GHz) and low temperature coefficient of resonant frequency ( less than xc2x125 ppm/xc2x0 C.) that can be suitably tuned for practical applications.